1. Field of the Invention
The present invention relates to an information processing medium for recording and reproducing information, and more particularly, it relates an information processing medium capable of preventing its skew caused by a change in ambient conditions and a lapse of time. The information processing mediums of the present invention include a medium for transmitting and creating information other than a medium for recording and reproducing information. Further, the present invention relates to an apparatus processing information by using such an information processing medium or an information processing apparatus having incorporated therein such an information processing medium.
2. Description of Related Art
FIG. 1 is a schematic sectional view illustrating a structure of an optical data recording medium. A conventional optical data recording medium is shown in a plan view of FIG. 8(a) and a side view of FIG. 8(b).
An optical data recording medium 10 comprises, as shown in FIG. 1 and FIGS. 8(a) and 8(b), a single layered or multilayered thin film layer 40 including at least any one of dielectric films 41, 43 (silicon nitride), a recording film 42 (TbFeCo) and a reflective film 44 (Al) formed by sputtering or the like on a disc-shaped substrate 20 made of a polycarbonate. On the thin film layer 40, a thin protective film 50 such as a resin film is formed. Further, a subject protective film 30 such as a resin film is formed on a light receiving surface of the substrate.
The substrate 20 is about 1.2 mm thick, the single layered or multilayered thin film layer 40 formed by sputtering is 10–300 nm thick, the thin protective film 50 is 1–30 μm thick, and the substrate protective film 30 is 1–30 μm thick. The polycarbonate substrate 20 constitutes almost the entire thickness of the optical data recording medium.
Therefore, rigidity of the medium substantially depends on that of the polycarbonate substrate 20. With the sufficient thickness of the polycarbonate substrate 20, deformation of the medium caused by a change in ambient conditions (temperature and humidity) is very small and there is no need to pay attention to a balance between stresses and bending moments generated in the layers.
In recent years, however, data recording and reproducing at high density on and from the optical data recording medium have been required. Accordingly, attempts to increase NA of an objective lens and decrease the substrate thickness have been made for reducing a beam spot diameter.
In general, the effective diameter (γ) of a laser beam incident on a disc medium is expressed as γ∝λ/NA with use of the wavelength (λ) of the laser beam and the NA of the objective lens.
In order to record information at a high density, it is necessary to reduce the wavelength λ of the laser beam and use an objective lens having a high NA. Merely increasing the NA results in coma that, when an angle formed by the objective lens with the disc is shifted, adversely affects recording. Coma is known to increase in proportion to the cube of the NA. To prevent coma, the thickness of the substrate needs to be set to 1/(NA)3.
The thickness of the substrate shows a tendency to decrease from a conventional dimension of 1.2 mm to an almost half or less, i.e., 0.6 mm or 0.5 mm so that information is recorded at a high density.
In such a case, the rigidity of the optical data recording medium depends upon not only that of the polycarbonate substrate 20 but also the stresses or bending moments generated in the layers. Such a medium will remarkably be warped if the ambient conditions (temperature and humidity) are changed. Therefore, it is important to establish an appropriate balance between the thicknesses and the like of the layers.
Japanese Unexamined Patent Publication No. Hei 4(1992)-195745 proposes a method of forming a dielectric film on a back surface of the substrate (where the thin film layer is not formed) for preventing the skew.
FIG. 9 shows a sectional view of an optical data recording medium according to the above publication. Components identical to those shown in FIG. 1 are indicated by the same reference numerals.
As shown in FIG. 9, a dielectric layer 60 is formed on a light receiving surface of a transparent polycarbonate substrate 20. The skew of the optical data recording medium is prevented by equalizing thermal expansion coefficients of a first dielectric film 41, a recording film 42 and a second dielectric film 43 formed on a surface opposite to the light receiving surface of the transparent substrate 20 with the thermal expansion coefficient of the dielectric layer 60 on the light receiving surface of the substrate.
Japanese Unexamined Patent Publication No. Hei 10(1998)-64119 describes forming the thin protective film 50 thick to reduce the skew caused by increase of the temperature of an optical disc. The constitution of this optical data recording medium is the same as that shown in FIG. 1. On the polycarbonate substrate 20, the thin film layer 40 is formed and then the thin protective layer 50 is formed to a thickness of about 30 μm to 50 μm. The increased thickness of the thin protective layer 50 is intended for establishment of a balanced with the thermal expansion coefficient of the polycarbonate substrate 20 so as to reduce the skew of the disc.
FIG. 10 shows a sectional view of another conventional optical data recording medium according to Japanese Unexamined Patent Publication No. Hei 4(1992)-364248.
The recording medium comprises a substrate 20, a thin film layer 40, a thin protective film 50 and a substrate protective film (a dielectric layer) 30. For preventing the skew caused by a change in humidity, an anti-permeation film 70 made of SiO2 or AlN is formed between the substrate 20 and the substrate protective film 30.
In the above-mentioned conventional recording media according to both of the publications, the dielectric layer (30, 60) must be formed by sputtering or the like on the light receiving surface of the substrate. Accordingly, in the manufacture thereof, the thin film layer 40 is formed on a surface of the substrate and then the substrate is turned over to form the dielectric layer (60, 30) on an opposite surface. Therefore, the manufacture is complicated and the charge of the manufacture facility is raised, which increases the manufacture cost.
Further, in a method described in Japanese Unexamined Patent Publication No. Hei 10(1998)-64119, the thickness of a thin protective film 50 is too large so that the costs are increased, which complicates the manufacture.
Where the optical data recording medium is a magneto-optic data recording medium, it is desirable to bring a magnetic head coil adjacent to the thin film layer 40 in order to reduce the magnetic field and inductance of the magnetic head coil to reverse a magnetic field at high speed during data recording. Therefore, the thick protective film 50 results in the reduction of magnetic properties of the magneto-optic data recording medium and causes problems in the data recording and reproducing.